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Abstract: The research activity reported in the present paper aims to evaluate the Forming Limit Curves (FLC) of the Mg alloy AZ31 in warm conditions (200°C) while keeping the equivalent strain rate constant. Specific tools to carry out such a formability test were designed and created: a flat punch (in line with Marciniak’s test) embedding a heating system was adopted in order to heat the central part of the specimen both rapidly and uniformly, where ruptures were forced due to the presence of a driving sheet between the specimen and the punch. A Digital Image Correlation system was also embedded in the formability equipment in order to acquire major and minor strains continuously and evaluate the moment and location of failures. Finite Element simulations were run in order to define punch speed profiles (which differ according to the geometry of the specimen) that were able to keep a constant equivalent strain rate in the region where ruptures were forced. Experimental tests implementing the punch speed profiles were carried out in order to obtain temperature, load and strain data. FLCs at two different strain rate levels (0.02s−1 and 0.002s−1) both confirmed and allowed us to quantify the noticeable strain rate effect of such an alloy on the FLC at a temperature of 200°C. The proposed approach for FLC evaluation is effective for materials whose properties are strongly influenced by the strain rate. Such FLC data can be usefully implemented in numerical simulations of sheet metal forming processes: while tensile tests can be used to determine variations in mechanical behaviour according to the strain rate, the FLCs evaluated in this work allow us to determine the occurrence of strain path-dependent critical conditions according to the strain rate. [Copyright &y& Elsevier]

Abstract: The present work aims at studying the tangential bending process (wiping) and the combined effect of a bending and stretching stress on thin (0.7mm) magnesium alloy (AZ31) sheets when working in warm conditions. The test equipment was designed in order to heat the sheet only in the bending region and to stretch the sheet after the wiping process; it was used for investigating the parameters affecting the stretch-bending state of stress in the sheet, quite common in the stamping process. A preliminary screening analysis over a large set of process parameter was performed using a full factorial design. As a result, only the effect of the temperature, the speed and the bend radius were further investigated according to a central composite design and using the springback factor and the maximum stretching load as response variables. The present work can give useful guidelines in the warm forming of AZ31 Mg alloy sheets in terms of key process parameters, fillet radii of the equipment, material stretch-bending strength (according to temperature, bending radius and speed) and precision of the final part (springback amount). [Copyright &y& Elsevier]

Abstract: The effect of laser cutting parameters on the mechanical behavior of laser butt welded joints whose edges were obtained by laser cutting was investigated. The paper aims to demonstrate that new high power solid-state fiber lasers not only represent a valid and reliable alternative to the most established CO2 and Nd:YAG laser sources, but also allow to obtain cuts having edges well suited for subsequent direct laser welding. First Ti6Al4V 1mm thick sheets having edges machined by milling were laser welded. Once the optimal welding condition was determined, the mechanical characterization of sheets cut by fiber laser and then laser welded was performed. Comparative strain analysis performed by a digital image correlation technique highlighted the effect of the gap between the sheets resulting from the different cut edge quality. Experimental results showed that the correct selection of laser cutting parameters allows to obtain butt joints characterised by mechanical properties comparable with the ones obtained by milling. Cutting edge quality in the optimal range of gap values allows to obtain the best mechanical performances of the joint. [Copyright &y& Elsevier]

Abstract: The aluminum/steel structural transition joints are widely used in shipbuilding industry due to the advantages of joining these two materials with important weight savings while exploiting their best properties. The research objective is the evaluation of process feasibility to laser weld structural transition joints to overcome manufacturing limitations of traditional TIG/MIG welding. This study presents results achieved during as-simulated laser welding by imposing severe thermal cycles to specimen obtained from structural transition joints with time periods longer than those normally recorded during laser welding. Metallurgical and mechanical characterization of heat treated specimen is performed to evaluate the influence of the thermal cycles on final joint properties. [Copyright &y& Elsevier]

In this work the single-pulse laser irradiation of a hypereutectoid steel was investigated using a fiber laser source, in a range of process parameters enabling surface hardening and remelting. Effects of laser power, pulse energy and defocusing distance were investigated using a numerical/experimental approach. Laser surface treatments were conducted on uncoated samples without any gas shielding, changing both the laser power and the pulse energy, and exploring a wide range of defocusing distances. Numerical simulations were conducted using a finite element model calibrated by means of an optimization procedure based on a specific calculation algorithm and using a subset of experimental data producing surface melting. Using both simulations and experiments, the process operating windows of the discrete spot laser treatment were determined: it was found that, when varying the laser power between 250 W and 750 W, melt-free hardened zones are produced with a maximum extension between 0.7 mm and 1.0 mm; on the contrary, in case of more tightly beam focusing conditions, surface melting occurred with a size of the re-melted areas ranging between 1.0 mm and 1.4 mm. Results further showed that a small change (generally 2–3 mm) of the defocusing distance suddenly brings the material from melting to a non-hardening condition. [ABSTRACT FROM AUTHOR]